Using immunostaining at the electron level, we plan to identify the organelles involved along the pathway of synthesis of the various procollagens and clarify whether the reactive substances are the procollagens or immunologically related substances. In the case of procollagen I, we have found by immunostaining and 3H-proline radioautography of odontoblasts that collagen precursors arise in the rough endoplasmic reticulum, migrate to the spherical distensions of Golgi saccules, which become cylindrical distensions, and in turn change to secretory granules. The sites at which the earliest precursor (pro Alpha chains) coil into the procollagen helix have not been decisively established. By preparing antibodies with more restricted specificity than those available so far and using them according to methods recently worked out for immunostaining at the electron microscope level, we hope to identify the sites containing only pro Alpha chains and those in which procollagen appear. Work done so far on the pathway of type I procollagen has been carried out on odontoblasts and osteoblasts. It is important to find out if the same pathway is followed in the main collagen producing cell, the fibroblast. In the case of procollagen III, the pathway of biosynthesis is unknown. In dental tissues, a few cells elaborate it, but at a low rate. We shall have to work with cells that are more active producers of type III if reactions are to be sufficiently intense to identify biosynthetic steps. A survey of tissues at various ages, will be carried out in the hope of finding active producers of type III. Finally, in the case of type IV, preliminary results show that rough endoplasmic reticulum and Golgi saccules are involved. Moreover, parietal yolk sac of rat and EHS tumor of mouse and known to produce type IV atively and will be investigated by the available methods.